Argon Ar

Discovery

The first hint of argon came in 1785, when Henry Cavendish repeatedly sparked air with extra oxygen to remove the nitrogen. A small bubble, about one part in 120, always remained, but its significance went unnoticed for more than a century.

In 1892 the physicist Lord Rayleigh found that nitrogen taken from air was consistently about 0.5 percent denser than nitrogen made from chemical compounds. Working with the chemist William Ramsay, he removed nitrogen and oxygen from air and in 1894 isolated a new gas whose spectrum matched no known element. It was the first noble gas to be discovered, and it forced chemists to add an entirely new column to the periodic table. Rayleigh received the Nobel Prize in Physics and Ramsay the Nobel Prize in Chemistry in 1904.

Origin of the name

The name comes from the Greek argos, meaning 'lazy' or 'inactive', because the gas refused to react with anything the discoverers tried. Its symbol was originally A, and it was changed to Ar in 1957.

Main uses

• Welding and metalworking: argon shields molten metal from oxygen and nitrogen in TIG and MIG welding, and it is used to refine stainless steel.
• Lighting: incandescent and fluorescent lamps contain argon to protect the filament or carry the discharge.
• Windows: argon between panes of double glazing improves insulation because it conducts heat less than air.
• Electronics: argon provides an inert atmosphere for growing silicon crystals and for sputtering thin films.
• Preservation: it protects wine, food and historic documents from oxidation.

Isotopes

Argon has three stable isotopes: argon-40 (about 99.6 percent of atmospheric argon), argon-36 and argon-38. Almost all argon-40 has been produced by the radioactive decay of potassium-40 in rocks over billions of years, which is why Earth's argon is almost all argon-40, whereas argon in the Sun is mostly argon-36. This decay forms the basis of potassium–argon and argon–argon dating, used to date volcanic rocks and the layers around human fossils. The radioactive argon-39, with a half-life of about 270 years, is used to date groundwater and ice.

In everyday life

Every breath you take contains about 0.93 percent argon, more than twenty times as much as carbon dioxide. It is harmless to breathe in normal amounts, but in enclosed spaces a leak of argon can displace oxygen without any warning smell, a known hazard in welding shops.

Large tanks of liquid argon are used in particle physics experiments searching for dark matter and studying neutrinos.